The four big moons of Jupiter probably are all about four and a half billion years old. But two of them look much younger. That’s because their surfaces are constantly repaved – one by fire, the other by ice.

The fire moon is Io. It’s the most volcanically active body in the solar system, with more than 400 volcanoes on its surface. Some of them release gas or lava, while others produce big explosions. That activity covers up any impact craters – the scars of collisions with big space rocks. Scientists have reported only one possible crater on Io – a good indication that the surface is young.

The ice moon is Europa. It has more craters than Io, but not a lot. That’s because it appears to have an ocean of liquid water below its icy crust. Water may ooze from the ocean, coating parts of the crust.

Some impacts by big space rocks may punch through the crust, allowing more water to reach the surface. And a recent study by scientists at the University of Texas found that an impact doesn’t have to go all the way through the ice. If it rams halfway through, it can melt enough ice for the warm water to melt the rest of the way to the ocean. That may allow some of the ocean water to reach the surface – helping Europa keep its “youthful” appearance.

Look for Jupiter near our own moon at dawn tomorrow. The planet looks like a brilliant star. Through binoculars, its big moons look like tiny stars arrayed near the planet.

Script by Damond Benningfield